U.S. patent application number 13/980589 was filed with the patent office on 2013-11-07 for electrical machine.
This patent application is currently assigned to ROLLS-ROYCE PLC. The applicant listed for this patent is Ellis FH Chong, Stephen J. Mountain. Invention is credited to Ellis FH Chong, Stephen J. Mountain.
Application Number | 20130292941 13/980589 |
Document ID | / |
Family ID | 43859485 |
Filed Date | 2013-11-07 |
United States Patent
Application |
20130292941 |
Kind Code |
A1 |
Mountain; Stephen J. ; et
al. |
November 7, 2013 |
ELECTRICAL MACHINE
Abstract
An electromagnetic machine, comprising: a first stator winding
having a first number of pole pairs; a second stator winding having
a second number of pole pairs which is different to the first
number of pole pairs; and, a modulator having a plurality of pole
pieces arranged relative to the first and second stator windings so
as to modulate the electromagnetic fields produced by the first and
second stator windings, thereby matching harmonic spectra of the
first and second stator windings.
Inventors: |
Mountain; Stephen J.;
(Derby, GB) ; Chong; Ellis FH; (Derby,
GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Mountain; Stephen J.
Chong; Ellis FH |
Derby
Derby |
|
GB
GB |
|
|
Assignee: |
ROLLS-ROYCE PLC
London
GB
|
Family ID: |
43859485 |
Appl. No.: |
13/980589 |
Filed: |
January 26, 2012 |
PCT Filed: |
January 26, 2012 |
PCT NO: |
PCT/EP2012/051206 |
371 Date: |
July 19, 2013 |
Current U.S.
Class: |
290/7 ; 310/46;
318/724; 322/59 |
Current CPC
Class: |
H02P 6/005 20130101;
H02K 16/04 20130101; H02K 7/1823 20130101; H02K 49/102 20130101;
H02P 9/14 20130101 |
Class at
Publication: |
290/7 ; 310/46;
322/59; 318/724 |
International
Class: |
H02K 16/04 20060101
H02K016/04; H02K 7/18 20060101 H02K007/18; H02P 6/00 20060101
H02P006/00; H02P 9/14 20060101 H02P009/14 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 16, 2011 |
GB |
1102682.0 |
Claims
1. An electromagnetic machine, comprising: a first stator winding
having a first number of pole pairs; a second stator winding having
a second number of pole pairs which is different to the first
number of pole pairs; and a modulator having a plurality of pole
pieces arranged relative to the first and second stator windings so
as to modulate the electromagnetic fields produced by the first and
second stator windings, thereby matching harmonic spectra of the
first and second stator windings wherein either the first or second
stator winding is positioned between the other of the first or
second stator winding and the modulator.
2. The electromagnetic machine as claimed in claim 1 wherein the
stator windings are fixed relative to each other.
3. The electromagnetic machine as claimed in claim 1 wherein the
modulator is rotatable and the first and second stator windings are
stationary relative to each other and the modulator.
4. The electromagnetic machine as claimed in claim 1 wherein the
first and second stator windings are proximate to one another.
5. The electromagnetic machine as claimed in claim 1 wherein the
first and second stator windings are located on a single
stator.
6. The electromagnetic machine as claimed in claim 1 wherein the
first and second stator windings are arranged so as to provide
radial flux with respect to the axis of rotation of the
modulator.
7. The electromagnetic machine as claimed in claim 1 wherein the
first and second stator windings are arranged so as to provide
axial flux with respect to the axis of rotation of the
modulator.
8. The electromagnetic machine as claimed in claim 1 wherein the
sum of the first and second stator pole pairs is the same as the
number of inter pole pieces.
9. An electrical system, comprising: the electromagnetic machine of
claim 1; a convertor connected to at least one of the first or
second windings; and, an electrical network connected to the other
of the first and second windings.
10. An electrical system as claimed in claim 11, wherein the
convertor is configured to supply a direct current to the winding
to which it is connected.
11. An electrical system as claimed in claim 10 further comprising
a gas turbine which is configured to rotate the modulator.
12. A method of controlling an electromagnetic machine as claimed
in claim 1, comprising the steps of: exciting the first stator
winding with a first current having a first frequency; and,
exciting the second stator winding with a second current having a
second frequency.
13. A method as claimed in claim 12 wherein either the first
frequency or the second frequency is substantially zero.
14. A method as claimed in claim 12 wherein the first current or
second current is provided by a convertor.
15. A method as claimed in a claim 12 further comprising:
monitoring the rotational position of the modulator relative to the
rotating magnetic field in the electrical machine; determining
whether it is within a predetermined speed range; and, adjusting
the first or second frequency to maintain the speed within the
predetermined range.
Description
[0001] The present invention relates to an electrical machine. In
particular, the invention relates to a variable speed electrical
motor, or a voltage regulated electrical generator.
[0002] There are many different known architectures of electrical
machine which can be used for variable speed drives or to generate
electricity from variable speed mechanical drives. The present
invention seeks to provide a new type of electrical machine
architecture.
[0003] In a first aspect, the present invention provides an
electromagnetic machine, comprising: a first stator winding having
a first number of pole pairs; a second stator winding having a
second number of pole pairs which is different to the first number
of pole pairs; and, a modulator having a plurality of pole pieces
arranged relative to the first and second stator windings so as to
modulate the electromagnetic fields produced by the first and
second stator windings, thereby matching harmonic spectra of the
first and second stator windings.
[0004] Having first and second stator windings and a modulator
allows a modulated magnetic field to be set up in the machine. The
modulation acts to match and lock asynchronous harmonics between
the spatially distributed electromagnetic fields of the first and
second stator windings thereby providing a geared synchronous
electromagnetic field between the windings. Providing one or both
of the first and second stator windings with a rotating magnetic
field thus results in the rotation of the modulator in a geared
manner, or a generating function if the modulator is rotated.
[0005] The geared nature of the machine means that a fixed
frequency power generation can be achieved from a variable speed
input without the need for a fully rated power converter.
Furthermore, because the first and second stator windings are
stationary, there are no rotating electrical windings and no
sliprings. Furthermore, there are neither permanent magnets nor an
exciter assembly as may ordinarily be required for a conventional
synchronous machine. The electrical machine can also be used as a
variable speed motor in which one of the windings can provide
geared speed control. This also prevents the need for a full rated
power convertor for a given speed range.
[0006] The modulator can include high permeability regions and low
permeability regions. The high permeability regions may have a
permeability greater than 100. Preferably, high permeability
regions are greater than 1000. The high permeability regions can be
pole pieces. The pole pieces can be laminated. The pole pieces can
be steel. The low permeability regions may be air. Preferably, the
low permeability regions are used for structural support of the
stator components using suitable low permeability and low remanence
materials.
[0007] The modulator can be positioned between the first and second
stator windings. Alternatively, either the first or second stator
winding can be positioned between the other of the first or second
stator winding and the modulator. The first and second stator
windings can be proximate to one another. Preferably, the first and
second stator windings are located on the same stator. The first
and second stator windings may be located on a common ferromagnetic
flux guide. The first and second stator windings can have
substantially no air gap therebetween. Preferably, the first and
second stator windings will be separated by a low permeability
layer which provides structural support.
[0008] The first and second stator windings form part of first and
second stators respectively. The first and second stators may
include flux guides. The flux guides may be ferromagnetic. The
ferromagnetic flux guides may include stator poles. Either or both
of the first and second stators may include slots which receive the
respective first and seconds stator windings. The slots may be
uniform in cross section. Alternatively, the slots may taper
towards the pole face of the stator pole pieces. In this way, the
stator poles may be flared.
[0009] The stator flux guides and or the stator poles may be
laminated. Preferably, the stator poles have a high magnetic
permeability.
[0010] The stator flux guides may include back iron portions. The
back iron portions may provide a high permeability envelope in
which the first stator winding, the second stator winding and the
modulator are located. The back iron portions may be an integral
part of the either or both of the first and second stators, or part
of either or one of the first and second stators, and the
modulator.
[0011] Preferably, the modulator is rotatable and the first and
second stator windings are stationary relative to the modulator and
to each other. The first and second stator windings may be part of
a first and second stator. The first and second stators may be
fixed relative to one another. The first and second stators may be
fixed relative to one another via a housing of the electrical
machine. The housing may include the back iron portion.
[0012] The electromagnetic machine may be a radial flux machine.
That is, the first and second stator windings can be arranged so as
to provide radial flux with respect to the axis of rotation of the
modulator. Alternatively, the electromagnetic machine may be an
axial flux machine. That is, the first and second stator windings
may be arranged so as to provide axial flux with respect to the
axis of rotation of the modulator.
[0013] Preferably, the pole pairs of the first and second stator
windings and the pole pieces of the modulator have a predetermined
ratio which allows the modulation of the electromagnetic field.
Preferably, the predetermined ratio adheres to the condition that
the sum of the first and second stator pole pairs is the same as
the number of pole pieces.
[0014] In a second aspect, the present invention provides an
electrical system, comprising: the electromagnetic machine of the
first aspect; a convertor connected to at least one of the first or
second windings; and, an electrical network connected to the other
of the first and second windings.
[0015] The electrical network may be a mains grid. The mains grid
may include a stiff network having a high electrical inertia. The
stiff network may have a substantially constant voltage. The stiff
network may have a substantially constant frequency.
[0016] Alternatively, the electrical network may be an isolated
grid having a low inertia. The isolated network may have a variable
frequency. The isolated network may have a variable voltage. The
isolated grid may include between one and ten electrical
generators. The generators may be low power. For example, the
generators may have rated power outputs of below 1 MW each. The
isolated grid may include a plurality of wind or tidal turbines
which feed into a mains grid. The isolated grid may be on an
aircraft or other vessel such as a ship.
[0017] The electromagnetic machine may be operated as a motor or a
generator. The electromagnetic machine may be coupled to a
mechanical drive. The mechanical drive may be coupled to the
modulator. The mechanical drive may comprise a gas turbine engine.
The mechanical drive may be a shaft of a gas turbine engine or any
rotating power system such as wind or tidal turbine. The shaft may
be an intermediate compressor shaft of a gas turbine engine.
Alternatively, the modulator may be coupled to a mechanical
load.
[0018] In a third aspect, the present invention provides a method
of controlling an electromagnetic machine according to the first
aspect, comprising the steps of: exciting the first stator winding
with a first current having a first frequency; and, exciting the
second stator winding with a second current having a second
frequency.
[0019] The first frequency or the second frequency may be zero.
That is, the first or second current may be substantially direct
current, DC. The first current or second current may be provided by
a convertor. The first current or second current may be provided by
a network.
[0020] The method may further comprise: monitoring the rotational
speed of the modulator; determining whether it is within a
predetermined speed range; and, adjusting the first or second
frequency to maintain the speed within the predetermined range.
[0021] The method may further comprise the steps of: monitoring the
voltage at the terminals of the electrical machine; determining if
the voltage falls outside of a predetermined range; and, adjusting
the first or second current to maintain the voltage at the
terminals of the electrical machine to within the predetermined
tolerance.
[0022] The electrical machine may be operated as a generator. The
electrical machine may be operated as a motor.
[0023] Embodiments of the invention are described below with the
aid of the following drawings in which:
[0024] FIG. 1 shows a schematic representation of an electrical
machine.
[0025] FIG. 2 shows a cross section of a second electrical
machine.
[0026] FIG. 3 shows a schematic representation of a third
electrical machine.
[0027] FIG. 4 shows a cross section of a fourth electrical
machine.
[0028] FIG. 5 shows an axial flux electrical machine.
[0029] FIG. 6 shows a detailed cross section of a stator
winding.
[0030] FIG. 7 shows a schematic representation of an electrical
system incorporating the electric machine.
[0031] FIG. 8 shows a known magnetic gearbox arrangement which is
included to aid with the explanation of the present invention
only.
[0032] Thus, in FIG. 1 there is shown a schematic of an electrical
machine 10 according to the present invention. The electrical
machine 10 can be operated as a motor or a generator, each of which
are described in more detail below.
[0033] The electrical machine 10 has a radial flux arrangement
which includes an inner, first stator 12 which carries a first
stator winding 14 in the form of a power winding, and a second,
outer stator 16 which carries a second stator winding 18 in the
form of a control winding and which opposes the first stator 12.
Although the inner and outer windings are denoted as the power and
control windings here, it will be appreciated from the following
description that they are interchangeable.
[0034] The first and second stator windings 14, 18 are supported
and partially surrounded by flux guides in the form of
ferromagnetic poles 20, 22. The ferromagnetic poles include pole
faces 24, 26 and back iron 28, 30 portions and are fixed relative
to each other via a housing (not shown in FIG. 1) of the electrical
machine 10. A modulator 32 is rotatably mounted coaxially with the
rotational axis 34 of the electrical machine 10 and includes a
plurality of high permeability portions in the form of
ferromagnetic pole pieces 36 positioned between the first and
second stator windings 14, 18.
[0035] The pole pieces 34 are substantially rectangular in radial
section and uniformly distributed in a cylindrical configuration
between the first and second stator windings 14, 18 with the
longitudinal axis of the pole pieces 34 extending parallel to the
rotational axis of the electrical machine 10. The pole pieces 34
are separated from the stator windings 14, 18 by respective first
and second air gaps 38, 40, and from each other by low permeability
portions 42 in the form of a non-magnetic material which could be
air or any structural medium, for example, an epoxy such as PEEK
(PolytEther Ether Ketone) or carbon fibre. The skilled person will
appreciate that the modulator 32 will also include some form of
support structure (not shown in FIG. 1), particularly where the
pole pieces 36 are separated from each other by air.
[0036] The back iron portions 28, 30 define an envelope which forms
a boundary of the magnetic circuit of the electrical machine and in
which a modulated magnetic field can be set up in use. In this way,
the back iron portions 28, 30 are placed at the inner most and
outer most regions of the magnetically interacting parts of the
electrical machine 10. It will be understood by the skilled reader
that the term back iron encompasses any suitable high permeability
material and is not confined to iron.
[0037] The operation of the electrical machine 10 employs a known
magnetic modulation technique in which the modulator 32 is used to
modulate the electromagnetic fields of the first and second stator
windings 14, 18 so as to match their harmonic spectra. This
modulation technique has been effectively demonstrated in the prior
art in the technical field of magnetic gearboxes which utilises
permanent magnets to provide a contra-rotating or co-rotating
geared movement between two rotors.
[0038] FIG. 8 shows a schematic cross section of a known magnetic
gearbox 810 which is included to aid with the understanding of the
operation of invention. The magnetic gearbox 810 includes an inner
rotor 812, an outer rotor 814 and a modulator 816 which are
substantially cylindrical and mounted concentrically so as to
rotate relative to one another about a longitudinal axis of the
gearbox 810. In the example provided in FIG. 8, the modulator 816
is held in a stationary position with the outer rotor 814 and inner
rotor 812 coupled to a mechanical drive and load, respectively.
However, one of the inner 812 or outer 814 rotors could be fixed
with the modulator 816 being rotatable.
[0039] The inner 812 and outer 814 rotors include a plurality of
permanent magnets 818, 820 which are separated from the modulator
by respective air gaps 822, 824. The modulator 816 includes a
plurality of laminated high permeability pole pieces 826 which are
rectangular in cross section and evenly spaced about the
circumference of the modulator 816.
[0040] In operation, the pole pieces 826 modulate the spatial
magnetic field produced by the permanent magnets 818 of the inner
rotor 812 to provide a magnetic spectrum in the air gap 822
adjacent the outer rotor 814. The magnetic spectrum is modulated so
as to provide an asynchronous harmonic that matches the fundamental
harmonic of the outer rotor 814 permanent magnets 820. Thus, when
the inner rotor 812 is rotated, the outer rotor 814 is locked with
the matched, asynchronous harmonic of the modulated magnetic field
and thus rotated in a synchronous but geared manner. The modulation
is balanced in that the magnetic field produced by the outer rotor
814 is modulated to match the fundamental spectrum of the inner
rotor 812. Hence, if the outer rotor 814 is rotated, the inner
rotor 812 will also rotate in a geared manner.
[0041] The operation of typical magnetic gearboxes which utilise
harmonic spectra matching are further described in "A Novel
High-Performance Magnetic Gear", K Atallah, D Howe, IEE
transactions on magnetics, July 2001, and "A Novel "Pseudo"
Direct-Drive Brushless Permanent Magnet Machine" K Atallah, J Rens,
S Mezani, D Howe IEE transactions on magnetics, November 2008. This
technique has been further utilised in U.S. Pat. No. 6,794,781 and
GB2437568 which both describe different forms of electrical machine
which integrate a magnetic gearbox with a synchronous electrical
machine. All of these documents are incorporated by reference.
[0042] To put this in to the context of the present invention and
with reference to FIG. 1, there is shown an electromagnetic machine
10 where the mechanical rotation of the inner rotor 812 is
substituted by a travelling electromagnetic wave in the inner
stator 12 which can be created according to well known techniques
used in induction machines for example. The outer rotor 814 is
replaced by a spatially distributed wave provided by a direct
current in the outer stator 16. With this arrangement, the
modulator 32 will be caused to rotate due to the matching of the
harmonic spectra. The speed of rotation will be proportional to the
rotational speed of the travelling electromagnetic wave.
[0043] The rotational speed of the pole pieces 32 when used as a
motor is dependant on the excitation frequency, the number of pole
pairs in both the inner 12 and outer 16 stators and the number of
pole pieces.
[0044] Specifically, the gear ratio is given by:
.omega. inner .omega. pole_piece = p outer p inner + 1
##EQU00001##
[0045] where p.sub.outer is the number of pole pairs on the outer
stator, p.sub.inner is the number of pole pairs on the inner
.omega..sub.inner is the rotational speed of the field set up by
the inner winding and .omega..sub.pole.sub.--.sub.pieces is the
resultant speed of the modulator pole pieces 32.
[0046] The number of pole pieces is related to p.sub.outer and
p.sub.inner by:
n.sub.pole pieces=p.sub.inner+p.sub.outer
[0047] The electrical machine shown in FIG. 1 has a typical 3-phase
lap winding arrangement in the outer and inner stators which uses
one slot per pole per phase. The outer stator has sixteen poles
(eight pole-pairs) 18 and the inner stator has twelve poles (six
pole-pairs) 14 with a modulator having fourteen pole pieces 36 to
achieve a magnetic gear ratio of 2.33. The stator windings 14, 18
are each connected to an electrical source or network in a
conventional manner. The excitation provided to the windings from
the electrical source will be dependant on the type of operation
the machine is to perform which is explained further below.
[0048] FIG. 2 shows an axial cross section of an electrical machine
210 which is similar in construction to the machine shown in FIG.
1. Thus, the electrical machine 210 includes in radial series an
inner, first stator 212, a modulator 232 which is rotatable about
the longitudinal axis 234 of the electrical machine 210, and an
outer, second stator 216. The modulator 232 has a plurality of pole
pieces 236 which are separated from the first 212 and second 216
stators via respective air gaps 238, 240. The first 212 and second
216 stators include respective first 214 and second 218 stator
windings and ferromagnetic flux guides 220, 222.
[0049] The first and second stator windings 214, 218 are held in a
fixed relationship relative to one another via a housing 242. The
outer stator 216 is attached directly to the housing 242 and the
inner stator 212 is held on a cylindrical hollow support member
244.
[0050] The pole pieces 234 of the modulator 216 are positioned
between the first 214 and second 218 stator windings. The pole
pieces 236 are rectangular in radial section and axially extend
substantially parallel to the rotational axis 234 of the electrical
machine 210. The pole pieces 236 are supported via a support
structure 246 which includes first 248 and second 250 radial
flanges which extend perpendicularly from the shaft 252 of the
modulator 232. The modulator shaft 252 is co-axial with the
rotational axis 234 of the electrical machine 210 and rotatably
mounted to the housing 242 and inner stator support member 244 via
bearing sets 254a, 254b located either side of the first and second
radial flanges at either end of the modulator shaft 242.
[0051] The modulator 232 can be either mechanically driven or drive
depending on whether the electrical machine 210 is operating as a
generator or motor. Hence, a first end 256 of the modulator shaft
252 is arranged to be coupled to a mechanical drive for which
conventional means can be used.
[0052] The stator windings 214, 218 terminate in a terminal box 258
mounted on an exterior end face 260 of the housing 242, at the
opposite axial end to the first end of the modulator shaft 252. The
exterior end face 260 of the housing 242 includes apertures 262
through which respective wires from the first 214 and second 218
windings pass for connection. The conductors of an external
electrical source can enter the terminal box 258 via conventional
means as required by the chosen wiring system.
[0053] The terminal box 258 also houses ventilation means in the
form of two centrifugal fans 264 which provide the interior of the
machine housing 242 with a flow of cooling air through the
apertures 262 described above.
[0054] In order to provide control of the motoring and generating
functions, the electrical machine also includes a rotor position
encoder (not shown) which provides the positional information of
the rotor. The output of the encoder can be used by a controller to
determine what the position of the modulator is relative to the
rotating magnetic field. Hence, if there is a change in speed in
the machine, the frequency and phase of the control winding
excitation can be adjusted via a convertor to ensure that the
machine retains the generating or motoring function, as required.
The skilled person will appreciate that other control mechanisms
can be used to achieve the regulated operation of the electrical
machine.
[0055] FIGS. 3 and 4 show further embodiments of the invention. As
with the previous embodiments, the electrical machines 310, 410
shown in FIGS. 3 and 4 include first 314, 414 and second 318, 418
stator windings and a rotatable modulator 332, 432. However, in
these embodiments, the first 314, 414 and second 318, 418 stator
windings are placed proximate to each other with the modulator 332,
432 positioned inside the inner stator winding 314, 414. In this
way, the inner stator 314, 414 winding is located between the pole
pieces 336, 436 and outer stator winding 418 and does not include
an integral back iron portion. However, the inner circumferential
surface of the modulator includes a back iron portion 328, 428
thereby forming the envelope as described above and preventing
unnecessary flux leakage.
[0056] The electrical machine 310, 410 is shown as having an air
gap 338, 438 between the first 314 and second 414 stator windings
in FIGS. 3 and 4. Although the electrical machine 310, 410 will
theoretically operate with such an air gap 338, 438 it is
preferable to have little or no air gap between the stator windings
in order to prevent the associated reluctance and magnetic losses.
Hence, the first and second stators share a common ferromagnetic
flux guide which is formed as a single structure.
[0057] Although FIG. 4 does not show the terminal box or
ventilation features of the embodiment shown in FIG. 2, it will be
appreciated that these can be incorporated as required.
[0058] FIG. 5 shows a cross section of a further embodiment of the
electrical machine 510 of the invention. The electrical machine 510
of this embodiment has an axial flux arrangement. Thus, there are
first 514 and second 518 stator windings arranged in a plane which
is perpendicular to the axis of rotation 534 of the electrical
machine 514 so as to provide an electromagnetic field having flux
lines which run substantially parallel to the axis of rotation 534.
The modulator 532 extends perpendicularly from the modulator shaft
552 so as to be positioned between the stator windings 514, 518 and
orthogonal to the electromagnetic field produced by the windings
514, 518. The modulator shaft 532 is supported by the housing 542
of the electrical machine 510 via bearing sets 554 placed
equidistantly of the pole piece support structure. It will be
appreciated that the ratio of pole pairs of the stator windings and
pole pieces follow the same relationships described in the above
embodiments.
[0059] FIG. 6 shows a cross section of a portion of the inner
stator winding conductors 614 and pole face 624 of the
ferromagnetic flux guide beneath the pole pieces 636 of the
modulator. It can be seen that the stator teeth include tapered
flange portions 640 at the distal ends thereof so as to provide
flared pole faces 624. This configuration helps reduce the higher
order slotting harmonics in the windings 614 so as to better mimic
a permanent magnet arrangement. The skilled reader will appreciate
that the inner and outer stator can include flared pole
portions.
[0060] The electrical machines described above can be made using
conventional manufacturing techniques well known in the technical
field. Hence, for example, the housing can be made from steel and
the windings from copper. The pole pieces and ferromagnetic cores
are made from laminations of steel.
[0061] The use of alternating current in the inner and outer stator
windings allows the field strength within the machine to be
modified and the vector of the field to be rotated at given speed.
Thus, if one winding is connected directly to an electrical network
the voltage and frequency are defined for that winding. The second
winding can then be used to establish field strength needed to
maintain voltage equilibrium with the frequency and phase on the
winding used to impose the torque relationship required upon the
modulator. The result is an electrical machine that is able to
achieve variable speed operation whilst directly connected to the
electrical supply.
[0062] The electrical behaviour of the machine is generally similar
to a conventional synchronous machine and the control methods used
on synchronous machines are generally applicable to the electrical
machine of the present invention.
[0063] FIG. 7 shows a schematic of an electrical system 710 in
which the electrical machine is used. The system includes: the
electrical machine 712 of the invention having a power winding, a
control winding and a modulator; an electrical source in the form
of a convertor 714; and a network 716. The modulator of the
electrical machine 712 is connected to a rotatable mechanical load
or drive (not shown) via a shaft 718.
[0064] In the system shown in FIG. 7, the network 716 is connected
to the power winding and the convertor 714. The convertor 714 is
configured to excite the control winding with an electrical
frequency within a range extending from zero, i.e. direct current,
DC. The upper limit of the frequency range provided to the control
winding will depend on the gear ratio between the windings, the
frequency of the network and the required performance of machine in
terms of the rotational speed of the modulator.
[0065] The network 714 may have a substantially constant supply
frequency and voltage or may be variable in each instance. For
example, the electrical network 714 may be a 50 or 60 Hz mains grid
with a fixed frequency and rigid voltage, or could be an isolated
network in the form of an aircraft network requiring a frequency in
the range between 350 and 800 Hz.
[0066] Where the frequency in the power winding is substantially
fixed, the frequency of the control winding supply can be adjusted
to provide a different synchronous speed in the machine 712. For a
variable frequency network, the control winding supply can be
adjusted either to maintain a constant speed or provide a variable
speed, as required.
[0067] The mechanical shaft 718 can be connected to a mechanical
load when the machine 712 is to motor, or a mechanical drive when
the machine 712 is to be used as a generator. The source of
mechanical drive could be from a wind or tidal turbine, or a
combustion engine such as a gas turbine engine, although any
suitable source of mechanical drive could be used.
[0068] For a fixed speed, the control winding is provided with a
direct current, D.C., from the convertor 714. The power winding is
connected to the network 716. To operate the machine 712 as a
generator in this configuration, the modulator is driven through
the shaft 718 by a suitable drive as described above. The DC in the
control winding sets up a magnetic field which interacts with the
rotating modulator to provide a rotating magnetic field which
induces a voltage in the power winding which is connected to a
load, via the network.
[0069] In the case of a preferred embodiment where the machine 712
is employed within an aircraft generation system, the voltage in
the network can be regulated by increasing the magnitude of the
voltage in the control winding which alters the field strength. If
the machine 712 is connected to a fixed voltage network 716, then
altering field strength can be used to control the reactive power
in the network 716.
[0070] For a variable speed operation, the control winding is fed
with AC. The frequency of the AC will depend on the gear ratio
between the windings, the frequency of the network 716, and the
required performance of machine 712 in terms of the rotational
speed of the modulator.
[0071] The connection of the mechanical drive 718 and load
notwithstanding, when the control winding is fed with an AC current
the convertor 714 can be adapted to control the power factor of the
excitation provided to the control winding. Thus, as is generally
known in the art for synchronous machines, for a fixed load at a
given speed the electrical machine 712 can either be motoring or
generating depending on the phase control of the excitation current
in the control winding. Hence, if there is a change in speed in the
machine 712, the frequency and phase of the control winding
excitation can be adjusted via a convertor 714 to ensure that the
machine 712 retains the generating or motoring function, as
required.
* * * * *